Extreme ultraviolet light, e.g., electromagnetic radiation having a wavelength of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers. Here and elsewhere herein the term “light” is used even though it is to be understood that the radiation described using that term may not be in the visible part of the spectrum.
Methods for generating EUV light include converting a target material from a liquid state into a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium, tin, or some other material, with one or more emission lines in the EUV part of the spectrum. In one such method, often termed laser produced plasma (“LPP”), the required plasma is produced by using a laser beam to irradiate and so to vaporize a target material having the required line-emitting element to form a plasma in an irradiation region.
The target material may take many forms. It may be solid or a molten. If molten, it may be dispensed in several different manners such as in a continuous stream or as a stream of discrete droplets. As an example, the target material in the discussion which follows is molten tin which is dispensed as a stream of discrete droplets. It will be understood by one of ordinary skill in the art, however, that other target materials, physical phases of target materials, and delivery modes for target materials may be used.
The energetic radiation generated during de-excitation and recombination of ions in the plasma propagates from the plasma omnidirectionally. In one common arrangement, an EUV optical element in the form of a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct, and, in some arrangements, focus the light to an intermediate location. The collected light may then be relayed from the intermediate location to where it is to be used, for example, to a set of scanner optics and ultimately to a wafer in the case where the EUV radiation is to be used for semiconductor photolithography.
The target material is introduced into the irradiation region by a target material dispenser. The target material dispenser is supplied with target material in a liquid or solid form. If supplied with target material in a solid form the target material dispenser melts the target material. The target material dispenser then dispenses the molten target material into the vacuum chamber containing the irradiation region.
The process of vaporizing the target material creates debris. This debris can degrade the reflectivity of the collector if the debris is allowed to reach the collector surface. In some systems H2 gas at pressures in the range of 0.5 to 3 mbar is used in the vacuum chamber for debris mitigation. In the absence of gas, at vacuum pressure, it would be difficult if not impossible to protect the collector adequately from target material debris ejected from the plasma. Hydrogen is relatively transparent to EUV radiation having a wavelength of about 13.5 nm and so is preferred over other candidate gases such as He, Ar or other gases which exhibit higher absorption at a wavelength of about 13.5 nm.
H2 gas is introduced into the vacuum chamber to decelerate the energetic debris (ions, atoms, and clusters) of target material created by the plasma. The debris is decelerated by collisions with the gas molecules. For this purpose a flow of H2 gas counter to the debris trajectory is used. This serves to reduce the damage caused by deposition, implantation, and/or sputtering of target material on and into the optical coating of the collector. Using this method it is believed possible to decelerate energetic particles with energies of several keV down to the thermal energy of the gas by the many gas collisions at these pressures over the distance between the plasma site and the collector surface.
Another reason for introducing H2 gas into the vacuum chamber is to facilitate cleaning of the collector surface. The H2 gas may be dissociated into hydrogen radicals H*. The hydrogen radicals H* in turn help to remove target material deposits from the collector surface. For example, in the case of tin as the target material, the hydrogen radicals participate in reactions on the collector surface that lead to the formation of volatile gaseous stannane (SnH4) which can be pumped away. For this chemical path to be efficient it is preferred that there is a low H recombination rate (the rate at which the radicals recombine to form H2 molecules) on the collector surface so that the hydrogen radicals are available instead for combining with the Sn to form SnH4. Generally, non-metallic compounds like nitrides, carbides, borides and oxides have a lower H recombination rate as compared to pure metals.
As mentioned, one measure for protecting the surface of the collector 30 (FIG. 2) from debris from the irradiation site 28 involves causing a flow of a gas such as molecular hydrogen across the collector surface. This gas flow deflects the debris so that the flux of debris onto the surface of the collector 30 is reduced. It is preferable that the gas flow be uniform across the collector surface so that the entire collector surface can benefit substantially equally from the protection afforded by the gas flow. In designs in which gas flow is distributed from both the center and the perimeter of the collector, it is necessary for the gas to travel more than 300mm to maintain flow across the collector surface. This requirement makes it difficult to maintain a uniform flow. Other designs, such as a “shower head” collector, deliver gas flow perpendicular to the collector surface and so do not require maintaining a uniform parallel flow, but also do not provide the benefits of a flow that is tangential or parallel to the collector surface.
There is thus a need for a gas delivery system that will introduce a flow of gas that is parallel to the surface of the collector in a manner that facilitates a uniform flow across the collector surface.